grape_oauth2_test.rb

# Gemfile

source 'https://rubygems.org'

gem 'grape_oauth2', git: 'https://github.com/nbulaj/grape_oauth2.git'

gem 'grape', '~> 1.0'
gem 'rack-oauth2'

gem 'tzinfo-data', platforms: %i[mingw mswin x64_mingw jruby]

# config.ru

require 'bundler/setup'
Bundler.setup

require_relative 'api'

run API

# api.rb

require 'grape_oauth2'

Grape::OAuth2.configure do |config|
  config.client_class_name = 'Application'
  config.access_token_class_name = 'AccessToken'
  config.resource_owner_class_name = 'User'
  config.access_grant_class_name = 'AccessCode'

  config.realm = 'Custom Realm'

  config.allowed_grant_types << 'refresh_token'
end

class Comments < Grape::API
  resources :comments do
    get do
      { value: 'test' }
    end

    get :protected do
      { value: 'works '}
    end
  end
end

class Posts < Grape::API
  before { access_token_required! }

  resources :posts do
    get do
      { value: 'test' }
    end

    get :protected do
      { value: 'works '}
    end
  end
end

class API < Grape::API
  version 'v2', using: :path
  format :json

  include Grape::OAuth2.api
  mount Comments
  mount Posts
end

# run application

$ rackup config.ru 
# [2017-12-22 10:52:20] INFO  WEBrick 1.3.1
# [2017-12-22 10:52:20] INFO  ruby 2.4.1 (2017-03-22) [x86_64-linux]
# [2017-12-22 10:52:20] INFO  WEBrick::HTTPServer#start: pid=2766 port=9292

$ curl -GET 'http://localhost:9292/v2/comments.json'
# {"value":"test"}

$ curl -GET 'http://localhost:9292/v2/posts.json'
# {"error":"unauthorized"}

$ curl -GET 'http://localhost:9292/v2/posts.json?access_token=123123'

# ...
# NameError at /v2/posts.json
# uninitialized constant AccessToken

# 127.0.0.1 - - [22/Dec/2017:10:52:24 +0300] "GET /v2/comment HTTP/1.1" 404 - 0.0313
# 127.0.0.1 - - [22/Dec/2017:10:52:27 +0300] "GET /v2/comments HTTP/1.1" 200 16 0.0133
# 127.0.0.1 - - [22/Dec/2017:10:52:32 +0300] "GET /v2/posts HTTP/1.1" 401 24 0.0442
# 127.0.0.1 - - [22/Dec/2017:10:52:42 +0300] "GET /v2/posts.json HTTP/1.1" 401 24 0.0008